Osteoarthritis of the knee is more painful and more damaging in smokers, a study reports.

Men who smoked had more progressive disease and reported higher levels of pain, a US team found.

Around a million people suffer from osteoarthritis in the UK - a condition causing inflammation and loss of cartilage in the joints.

The study in Annals of Rheumatic Diseases supports previous research showing smokers feel more back pain.

The researchers followed 159 men with symptomatic osteoarthritis of the knee for 30 months.

It's not unique to knees, there's a strong relationship with smoking and worse back pain

Professor David Felson

Overall, 12% of the participants were current smokers.

MRI scans of the knee showed that the smokers had a more than two-fold increased risk of loss of cartilage in the knee joint - a process that occurs as the disease progresses.

Men who smoked also had higher pain scores than men who didn't smoke throughout the study.

The greater amount of pain was unlikely to be due to increased cartilage loss as cartilage does not have pain fibres.

Pain threshold

Study author Professor David Felson, professor of medicine at Boston University Medical School said there were a few potential explanations for the pain finding but it could be explained by changes in pain thresholds in smokers.

"There is data elsewhere that shows smokers feel more pain. It's not unique to knees, there's a strong relationship with smoking and worse back pain.

"My guess is it's a general increase in musculoskeletal pain and that something in cigarette smoke sensitises people to lower pain thresholds," he said.

Osteoarthritis is much more common in women but there were too few women smokers in the study to measure the effect on them.

However Professor Felson said the results would probably be the same: "There's no reason it would be different in women as I don't think the biology is likely to be different but we can't be sure."

"It's an additional reason to stop smoking as it may lessen the pain and rate of cartilage loss," he added.

Dr Peter Stott, a GP in Tadworth, Surrey and member of the scientific advisory committee of the National Osteoporosis Society said it was hard to be categorical about the findings as the research was preliminary but it was another reason to stop smoking.

"It's interesting but there could be a number of reasons for the findings which are unrelated to smoking.

"One is that smokers are different emotionally as they have a tendency to be addicted and to need things that give them relief such as cigarette smoke.

He added: "The finding that needs to be looked at in another study is the cartilage loss because other studies have shown smoking is protective."